now when the dead guys were building these compressors, they did some math on a cocktail napkin while eating a cheeseburger at lunch, went back to the lab and plugged in a few values for R and C.
then they probably measured the response time with a scope and tweaked the thing to get it to sound good,
the fairchild has some combination RC networks which are interesting,
music usually has a complex wave form, so computing the rms voltage coming out of a compressor bridge circuit is difficult,
you can use RC combos that are good for cymbal crashes, or ones that slowly react to bass, somewhere in between would be vocals,
Rein Narma told me that he used to put on orchestra music while tuning up the 660,
since they use those cymbals all the time, you want a quick response time to catch a cymbal crash, but for overall compressing, you do not want an abrupt level change in the music, so you use a quick response for the high end transients, and a slower response to average the level.
so you put a small cap for cymbals, and a bigger cap for average material, side by side, with resistors to balance how much compressing on which band, so you get 2 response times for the same setting, then you use your ear to get the final values for R and C.